I'm partially familiar with resistors and I'm trying to find an expert.

I bought a LED and I need a resistor for it but I'm not sure what size. The guy I bought it from said 5ohm 3watt but I can't find that resistor and I have also used some of the online resistance calculators and they didn't say to use that resistor.

Well I'm trying to stay as cheap as possible because i'm gonna be building this project in bulk, however can this buckpuck power multiple LEDs?

I was thinking if I could hook this up directly at the power source then I could run all the LED powers to this buckpuck and I would only need one which wouldn't be that bad being that these, correct me if i'm wrong, adjust to changes in supply voltage???

The BuckPucks automatically adjust the current to stay very close to their specified limits.

You really do not want to operate those LEDs using plain resistors in an automotive environment. System voltage can be as low as 10v when cranking the engine, and over 40v during transients. Your LED's intensity will vary quite a bit even under normal operation. Additionally, power dissipated in the resistor(s) will be wasted.

If you want something less expensive, look at building your own switcher using an IC. Texas Instruments, National Semiconductor, Linear Technology, and many other manufacturers make IC's specifically for LED current control.

Off the top of my head, you might look at a National LM3404, LM3404HV or LM3405.
Or, NXP's ZLD1366.
There are many, many others.

If you are considering selling these things, you won't have much of a market if they are not reliable and efficient.

Yes they can. Just look at the datasheets for the products. Some are for arrays. Some are for flashlight type applications where 1 led is on for "dim" mode and 7 leds are on for "bright" mode. There are ka-zilions of combinations for led drivers and leds.